The present disclosure relates to a high-tensile brass alloy and a product, made of a high-tensile brass alloy, subjected to friction load.
For typical friction applications in a lubricant environment, low coefficients of friction of the alloy used are generally required. Additionally, the coefficient of friction should be adaptable within predefined limits to the particular application, specifically the friction partner, the lubricant used, and the friction conditions, such as contact pressure and relative speed. In particular, this is true for piston sleeves, which are acted on by high static and dynamic loads, as well as for synchronizer rings. Furthermore, applications with high relative speeds of friction partners, as are present for axial bearings of a turbocharger, for example, require alloys which in addition to reduced heat generation also ensure good heat dissipation from the friction surface.
The friction power and the oil contact result in a tribological layer which has accumulated lubricant components on the bearing surface. A uniform, high deposition rate of the lubricant components and their breakdown products is necessary to obtain a sufficiently stable adsorption layer on the sliding layer.
A suitable material for a component that is used in an oil environment, such as a synchronizer ring or a bearing part for a bearing in such an environment, is additionally characterized by wide-ranging oil tolerance, so that the structure of the tribological layer is largely insensitive to the selection of certain oil additives. Additionally, a component made of such an alloy should have good emergency running properties, so that a sufficient service life, even under dry friction conditions, may be ensured.
For components under friction load, it is also important that the alloy used have sufficient strength. Accordingly, a high 0.2% yield strength should be present to minimize plastic deformations that occur under load. Nevertheless, such a component should have a certain degree of plastic deformation above the yield strength, to the point of failure.
Also, it is necessary for such components to be particularly hard and to have high tensile strength in order to increase their resistance to abrasive and adhesive stresses. At the same, there must be sufficient toughness as protection against impact stresses. In this regard, it is necessary to reduce the number of microdefects and retard the defect growth that develops therefrom. This is accompanied by the requirement of providing an alloy having a preferably high fracture toughness and which is largely free of internal stresses.
In many cases, suitable alloys for parts under friction load are high-tensile brasses, which in addition to copper and zinc as the primary components are alloyed with at least one of the elements nickel, iron, manganese, aluminum, silicon, titanium, or chromium. Silicon brasses in particular meet the requirements stated above; CuZn31Si1 represents a standard alloy for friction applications such as piston sleeves. Furthermore, it is known to use tin bronzes, which in addition to tin and copper additionally contain nickel, zinc, iron, and manganese, for friction applications or mining.
A brass alloy for use in turbocharger bearing applications is known from WO 2014/152619 A1. This brass alloy contains a large quantity of manganese, with 1.5 to 3.0 wt %, but only a small amount of Sn, in particular less than 0.4 wt %. This previously known brass alloy allows a maximum lead content of 0.1 wt %, to adhere to the stringent requirements for freedom from lead. However, it is favorable to incorporate lead as an alloy component in brass alloys, since this facilitates chip breaking, thus improving machining. In addition, lead is typically incorporated as a corrosion inhibitor in high-strength brass alloys, whose alloy products are used in an oil environment. This applies in particular to oil environments that come into contact with bioethanol. Bioethanol is contained in vehicular fuel, and passes into the motor oil, for example, due to leaks in the piston rings or other types of entrainment. This applies to vehicles that are primarily used for short trips, where the engine does not reach its operating temperature. The same applies for turbocharger bearings, which as a result of the bioethanol and its waste products contained in the exhaust gases are exposed to an aggressive mixture. Thus, an acidic environment develops in the oil. A lead sulfate surface layer forms from the sulfur contained in the oil together with the lead contained in the alloy product. This surface layer acts as a corrosion inhibitor, similar to a passivation layer.
The structure of such a brass alloy, which may have different phases in the matrix, also affects the mechanical load capacity and the corrosion resistance. Brass alloy products with a high proportion of the α phase are characterized by generally good corrosion resistance, high toughness and elongation at break, and good cold formability. It is disadvantageous that these types of alloy products have rather poor hot forming capability as well as low resistance to abrasion and adhesion. In contrast, brass alloy products having a β phase have high mechanical wear resistance, high strength, good heat formability, and low adhesion. A disadvantage of these alloy products, however, is their relatively poor cold formability, relatively low toughness, and much poorer corrosion resistance compared to a brass alloy product having an α phase. Although brass alloy products having a γ phase are characterized by good corrosion resistance and good mechanical wear resistance, they have low toughness and relatively low forming capability. Thus, it is apparent that, although each phase has advantages in one area or the other, disadvantages must be accepted.
For brass alloy products of the type under discussion which are used in an oil environment, corrosion also plays a role, as previously indicated. In this regard, an alloy from which alloy products are produced and used, for example in an oil environment with respect to an axial bearing, must also meet these requirements.
Under friction load, an adsorption layer that is composed primarily of lubricant additives develops on a workpiece made of a copper alloy, even after a short period of contact with the lubricant. Under thermomechanical stress, a reaction layer composed of mutually reactive components of the adsorption layer and alloy constituents near the surface forms beneath the adsorption layer. In the process, the adsorption layer and the reaction layer form an outer boundary layer on the copper alloy workpiece, below which an inner boundary layer several microns thick is situated. Due to its proximity to the outer boundary layer, the inner boundary layer is affected by the mechanical load that acts on the surface, as well as by the chemical conversion processes in the reaction layer. Diffusion processes and oxidation processes of the substrate alloy may influence formation of the reaction layer around the inner boundary layer.
Many lubricants contain additives, such as sulfur- and phosphorus-containing additives, which under appropriate thermomechanical stress due to frictional contact may have a corrosive effect, which in turn reduces the service life of a workpiece considerably. Copper alloys have already been proposed to reduce the corrosive effect of sulfur components in the lubricant. A copper alloy for the bearing of a turbocharger is known from JPS60162742 A, having a composition of 57-61% copper and 2.5-3.5% lead by weight, with iron and zinc possibly present as impurities. The aim is to form a stable CuS layer on the friction surface.
Additives are often added to lubricants with the aim of reducing the corrosion on a friction surface and decreasing the abrasive wear. One example of such a corrosion inhibitor (anti-wear active substance) is zinc dialkyl dithiophosphate. A phosphate glass that protects the surface forms in the reaction layer from this additive. To this end, ideally an exchange of the ligands of the additive with alloy elements as well as intercalation of substrate cations take place, so that a durable reaction layer forms. However, reaction processes that protect the surfaces are dependent on the composition of the inner boundary layer of the substrate material. Furthermore, additional additives in the adhesion layer may influence the process by having a competing effect, regarding adhesion, with the additives that protect the surfaces. Also, of significance for the layer formation and degradation processes are the alloy structure, thermal processes of the reaction layer in regards to heat dissipation, and localized temperature peaks. This could possibly even result in an undesirable chemical degradation process of the friction layer, with involvement of corrosion inhibitors, as a function of the particular tribological system that is present.
The foregoing examples of the related art and limitations therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.